THE LIFE CYCLE 43 



chief source of energy, especially in the lower animals. As already 

 pointed out, different constituents of the substratum show very 

 different degrees of stability, some being evanescent and disappear- 

 ing at once with slight change in conditions, while others once 

 formed persist for a long time or through life. It is therefore 

 impossible to distinguish sharply between what constitutes the 

 substratum and what does not. We can only say that the sub- 

 stratum consists in general of more stable substances than those 

 which do not appear in it. 



As our knowledge of the great complex of reactions which we 

 call metabolism increases, it becomes more and more evident that 

 the different reactions of the complex are not entirely independent 

 of each other, but constitute a reaction system. In this system the 

 oxidations appear to be the most important or dominant factor, the 

 independent variable, as Loeb and Wasteneys ('n) express it, upon 

 which the other reactions depend more or less closely. Rate of 

 oxidation is a more fundamental factor in growth than the amount 

 of nutritive material in excess of a certain minimum. From this 

 point of view the term " metabolism" loses some of its vagueness. 

 It is not simply a hodgepodge of chemical reactions in which now 

 one, now another, component is most conspicuous, as external con- 

 ditions change, but rather an orderly correlated series of events in 

 which certain reactions play the leading roles. The rate or char- 

 acter of component reactions may change very widely with external 

 conditions, but nevertheless the reaction system retains in general 

 certain definite characteristics and the relation between its com- 

 ponent reactions persists. Anabolism and katabolism, the synthesis 

 and the breakdown of the substance of organisms, are not independ- 

 ent processes, but the syntheses are apparently associated with, 

 and in greater or less degree dependent in some way upon, the 

 oxidations. 



From this point of view functional hypertrophy loses its peculiar 

 character. It is not in any sense a " regeneration in excess" or an 

 " over-compensation, ): ' as it is so generally assumed to be, but is 

 simply the result of increased metabolism in the presence of an 

 adequate nutritive supply. Increased metabolism under these 

 conditions means increased production of structural substances. 



